An epidemic form of severely dehydrating adult diarrhea is caused by a new and completely distinct human Grp B rotavirus called Adult Diarrheal RotaVirus (ADRV) (12, 48) .ADRV is responsible for outbreaks of adult diarrhea affecting 5-44% of the local Chinese population (46,48, 105). Our study of ADRV in the U.S. population indicates that 5% of individuals demonstrate serologic immunity to Grp B rotaviruses. We have since detected a second strain of ADRV, with a small plaque phenotype and a unique VP4 neutralization antigen; and a third isolate from a U.S. nursing home. Grp B rotaviruses are morphologically indistinguishable from Grp A rotavirus strains. However, Grp B rotaviruses do not cross hybridize or react immunologically with other rotavirus groups. In fact, the proteins encoded by ADRV genes are only distantly related to Grp A rotavirus proteins. A unique feature of Grp B rotaviruses is the formation of fused cells or syncytia during infection. Virtually no information is available on mechanisms of ADRV-induced cell fusion or viral neutralization. The inability to tissue culture-adapt ADRV has hindered the development of neutralization assays and the definition of neutralization antigens. We have made important progress toward circumventing this limitation by transiently infecting Mal04 cells with ADRV and demonstrating that virus- specific antibodies neutralize ADRV. The recent cultivation of a porcine group B rotavirus provides an important tool for studying Grp B rotavirus neutralization. The combination of these results permit us to propose studies defining the molecular determinants of ADRV neutralization. The protein responsible for ADRV-induced cell fusion is likely to play a critical role in the mechanisms of viral neutralization and ADRV pathogenesis. To define the molecular determinants of ADRV-induced cell fusion and viral neutralization, we have molecularly cloned nearly all of the ADRV genes (9/11) and expressed their encode proteins. The AD6Fl protein was found to be similar to the respiratory syncytial virus Fusion protein. This intriguing finding has compelled us to functionally define the ADRV Fusion protein(s) and to directly address the mechanism of Grp B rotavirus cell fusion and the involvement of the Fusion protein in viral neutralization. The specific aims of this project are: (1) Group B Rotavirus Neutralization. A. Developing Neutralization Assays for ADRV. Hypothesis: The ADRV Fusion protein and outer capsid proteins VP4 and VP7 are neutralizing antibody targets. B. Defining Neutralization Epitopes and Polypeptides of ADRV VP4, VP7 and F Proteins. (2) Cell Fusion A. Define the genetic and protein Determinants of ADRV-mediated syncytia formation. B. Determine the Mechanism of ADRV Cell Fusion using assays for the cell surface expression, fusion, fusion inhibition, multimerization and mutagenesis of the ADRV F protein.